Gasoline service stations normally have underground storage tanks (USTs) from which fuel is pumped to dispensers. A typical installation makes use of a unitized motor and pump (UMP) in the storage tank which operates using one or more impellers to pump gasoline or another liquid fuel to a distribution head located above the tank. The flow path for the fuel includes a vertical column pipe which extends from the pump to the distribution head. From the distribution head, the fuel is supplied to one or more dispensers, each of which may have multiple fueling positions. The fuel is then delivered to a customer's vehicle tank via a hose and nozzle at each fueling position.
Governmental regulations typically limit the flow rate of fuel at each nozzle, for example to 10 gallons per minute. Because service station owners have an interest in servicing customers as quickly as possible, they desire a fuel flow rate approaching this maximum.
Submersible pumps are often configured to operate the impeller(s) at a constant RPM even if fuel is not being dispensed. However, operating the pump at a fixed speed may create undesirable high pressures during low or no flow conditions. In particular, when the pump is on and all nozzles are open, pressure in the pump is relatively low. This causes low flow rates at each nozzle. Thus, in some installations two or more UMPs may be manifolded together to achieve higher flow rates when a large number of nozzles are simultaneously open. In any case, when the pump is on and less fuel is being dispensed (i.e., one or more nozzles is closed), the pressure in the pump will increase. This pressure during a stopped flow condition (i.e., all nozzles are closed) may be high enough to damage components of the fuel dispensing system.
One prior art solution involves using a variable speed drive (VSD) to control the speed of the impellers in a low or no flow condition. These systems employ some method of feedback to determine when to reduce the impeller speed. For example, a VSD may be provided with a pressure transducer or it may monitor the current delivered to the pump motor. However, the VSD and its associated feedback devices are complex and expensive.
FIG. 1 illustrates the above-described operating characteristics of a standard constant speed UMP, a manifolded constant speed UMP, and a UMP using a VSD as the number of nozzles in operation increases.
Another potential solution involves using a bypass valve to divert fuel back to the UST during low flow conditions, thereby limiting the pressure. However, this may interfere with existing devices required for leak detection. Specifically, environmental regulations require that USTs be monitored for leaks, and typically a liquid level float is provided for this purpose. The liquid level float is adapted to detect small changes in liquid level to identify potential leaks. Because diverting fuel back into the tank causes liquid surface disturbances, this solution could interfere with the float's operation.